PROJECT SUMMARY The overall goal of the proposed investigation, Defining the Sluggish Cognitive Tempo Phenotype in Children, is to carefully delineate the unique behavioral phenotype associated with the observed pattern of slowed cognitive processing and behavioral functioning characteristic of children with sluggish cognitive tempo (SCT). The proposed study will be the first to combine caregiver-reported measures of SCT symptoms with careful behavioral assessment of cognitive slowing and associated comorbidities in order to narrowly define the SCT phenotype. This work will provide a basis for future identification of biomarkers (e.g., structural and functional imaging, genetic contributions) of the SCT construct in children SCT symptoms include daydreaming, difficulty initiating and sustaining effort, lethargy, drowsiness, and physical underactivity, with the core symptoms?lethargy, underactivity, and slowness?clearly separable from ADHD. Although SCT is associated with functional impairment in children and adolescents, this functional impact reflects SCT as defined only by caregiver ratings; little work to date has examined associations between ratings of SCT and performance-based measures of sluggish/slowed mental processing or response speed and none of the published work to date has carefully examined the pattern (e.g., consistency) of slowed responding in SCT, to determine its distinctiveness from the pattern of variable responding seen in ADHD. Although SCT by itself appears to be prevalent in ~2.5% of the general child population, estimates of comorbidity with ADHD range from 30-60%. Slowed processing of mental information, a key feature of the SCT construct, may represent a behavioral ?polyphenotype? (i.e., a phenotype constituting core deficits of more than one disorder). As such, SCT is not a new category of psychopathology, but may reflect a dimensional construct that is not presently well-captured in the current conceptualization of attention or learning disorders. It is possible that the psychological makeup of SCT could account for comorbidity between disorders of attention and learning, may add unique variance in predictions of functional outcomes, and its genetic architecture could account for the observed genetic correlations between these disorders. Using a sample including both referred and non-referred children, we intend to 1) narrowly define the behavioral phenotype of SCT in children, using both rating scale and performance-based measures emphasizing motor speed, reaction time, naming speed, verbal fluency, academic fluency, as well as untimed intellectual and academic skills; 2) characterize the consistency of reaction time in SCT as contrasted with the variability commonly seen in ADHD; and 3) examine the associations among behaviorally defined SCT with ADHD, mood symptoms, and language-based learning disorders (i.e., dyslexia).